1. Field of the Invention
This invention pertains to improvements in electronic circuits, and, more particularly, to improvements in electronic circuits that provide a driving current to an inductive load and to provisions for increasing the breakdown voltage of the circuit beyond that of the reverse breakdown voltage of its bipolar driver transistor.
2. Description of the Relevant Art
With reference to FIG. 1, a circuit 10 that is typical of prior art circuits for providing drive current to an inductor or inductive load 11 is shown. The circuit 10 includes an NPN transistor 14 having its collector connected to a power supply that supplies a voltage, V.sub.CC. The emitter of the transistor 14 is connected to a node, N1 to which one end of an inductor 11 is also connected. Additional resistive and other loading elements are shown by a block 16, connected between the other end of the inductor 11 and a voltage, V.sub.S.
A switch 17 is connected between the node, N1, and ground. The switch 17 is controlled to be on or off by a signal applied to an input terminal 18. The switch 17, typically is provided by a transistor, such as an n-Channel MOS or bipolar NPN transistor, with the gate or base, respectively, connected to the input 18.
The circuit is controlled by a signal on an input 20 connected to the base of the NPN driver transistor 14. Thus, it can be seen that the arrangement of the transistor 14 and switch 17 provides a push-pull drive scheme to the inductor 11. When, however, the switch 17 is turned off, the inductor 11 will attempt to continue the current flowing within its coils in the same direction as it was flowing preceding the turn off of the switch 17. Therefore, the voltage at the node N1 may in fact exceed the power supply voltage V.sub.CC to a point at which the reverse breakdown voltage of the transistor 14 is exceeded.
Some circuits provide clamping or other circuit mechanisms to control the excess voltage generated by the action of the inductor 11 at node N1.
On the other hand, in many applications the increased voltage is allowed to climb as far as possible so that the inductor may discharge its energy faster. The discharge may be practically limited by the reverse breakdown voltage of transistor 14.
Two reverse breakdown voltages are of interest. The first, BV.sub.ECO, is the one which is obtained by raising (in an NPN transistor) the voltage of the emitter with respect to the voltage of the collector and leaving the base terminal unconnected. The second, BV.sub.EBO, is the one obtained by raising the potential of the emitter with respect to that of the base and leaving the collector floating or at a potential equal or higher than the base. The second is always higher than the first and often twice as large.
To allow the voltage on node N1 to raise above the supply, transistor 14 is usually left undriven, (i.e., high impedance) so that its base is floating and the voltage on node N1 cannot raise more than the BV.sub.ECO of 14 above the supply.